1. Field of the Invention
The present invention generally relates to a prism and an optical device using the same. More specifically, the present invention relates to a prism used as an optical element for narrowing the wavelength band in an ultraviolet laser light source, and to an optical device such as a resonator using such a prism.
2. Description of the Background Art
In recent years, owing to improved resolution in photolithography using light, semiconductor devices have been provided with a higher degree of integration and performance. The development of submicron lithography with use of a reducing projection aligner is, in one aspect, directed to reduction in the wavelength of light employed.
Lithography using an excimer laser as a light source with a short wavelength has been employed. More specifically, as steppers using a KrF laser light source with a wavelength of 248 nm are put into practical use and resolution in lithography improves, the memory storage capacity of a semiconductor integrated circuit device and the clock frequency of a CPU (Central Processing Unit) are remarkably increased. Most recently, a stepper using an ArF laser light source with a wavelength of 193 nm is being developed. This type of stepper enables fine patterning allowing a line width of as small as 0.13-0.11 xcexcm. Accordingly, to enable the manufacture a dynamic random access memory (DRAM) having a storage capacity of 1-Gbit, the practical use of the stepper using the ArF laser light source is expected.
The stepper requires, as an exposure light source, a monochromatic laser beam with an extremely small line width and, more specifically a laser beam with a laser oscillation output of at most 20 W, laser oscillation frequency of 4 kHz, and laser oscillation spectrum with a full width at half maximum of at most 0.5 pm. To obtain this type of laser beam, an enlargement prism is inserted in a resonator of the exposure light source along with a diffraction grating as an optical element for narrowing the wavelength band. This enables selection of dispersed laser beams with different wavelengths and enlargement of laser beam, so that a monochromatic output laser beam is obtained.
Under the circumstance, there is a need for a prism with high transmittance which is capable of narrowing the wavelength band of a laser light source with a wavelength of at most 200 nm such as an ArF laser light source without decreasing laser energy efficiency.
A structure of an antireflection film having an antireflection effect against incident light in a deep ultraviolet region with respect to an optical element has been proposed in Japanese Patent Laying-Open No. 10-253802, No. 10-268106, and No. 11-64604. However, these laid-open applications merely disclose a structure of an antireflection film with reduced reflectance, but not with high transmittance. In addition, although the above laid-open applications have proposed a structure of an antireflection film used for an optical element such as a lens in an optical system including a stepper, for example, they do not disclose a structure of an antireflection film which is most suitable for a specific optical element, i.e., a prism.
An object of the present invention is to provide a prism provided with an antireflection film with high transmittance capable of narrowing the wavelength band of a laser light source with a wavelength of at most 200 nm without lowering laser energy efficiency, and to an optical device using the same.
First of all, the present inventor has noted that transmittance T(%) can be expressed by the equation T(%)=100xe2x88x92xcex1(%)=R(%) (xcex1: absorptance, R: reflectance). According to the equation, absorptance xcex1 must be reduced along with reflectance R to achieve high transmittance T. Then, the present inventor has studied the structure of an antireflection film which provides not only low reflectance but also low absorptance. In the course of study, the inventor has found not only a precise refractive index n(xcex) but also a precise extinction coefficient (xcex) in the region with a wavelength xcex of at most 200 nm as optical constants of a material for the antireflection film.
As a result, the present inventor has found that a prism with high transmittance can be obtained by forming an antireflection film, formed of a combination of a layer including thorium fluoride (ThF4) with relatively high refractive index and a layer including aluminum fluoride (AlF3) with relatively low refractive index, on a base member including calcium fluoride (CaF2).
In addition, the present inventor has found that thorium fluoride (ThF4) and aluminum fluoride (AlF3) exhibit relatively high affinity with respect to calcium fluoride (CaF2) as compared with other film materials, and hence exhibit relatively high adhesion with respect to a prism base member including calcium fluoride (CaF2).
Therefore, the prism according to the present invention has a base member including calcium fluoride with first and second faces intersecting with each other, a first antireflection film formed on the first face of the base member, and a second antireflection film formed on the second face of the base member. Each of the first and second antireflection films has a high-refractive-index layer with a relatively high refractive index, and a low-refractive-index layer with a relatively low refractive index. The high-refractive-index layer includes thorium fluoride, and the low-refractive-index layer includes aluminum fluoride.
In the above mentioned prism, in the case where a laser beam is incident upon the first face and emitted from the second face of the base member, there would not be a considerable decrease in laser energy efficiency when the laser beam passes through the first and second antireflection films because of high transmittance of the first and second antireflection films. Thus, the prism with high transmittance can be obtained.
In the above mentioned prism, the outermost surfaces of the first and second antireflection films preferably include aluminum fluoride. In this case, the outermost layer including aluminum fluoride serves as a protection layer against the moisture environment in which the prism is located. Thus, the prism with excellent environmental resistance can be obtained.
Further, in the above mentioned prism, each of the first and second antireflection films preferably include a high-refractive-index layer and low-refractive-index layer which are alternately layered.
In the prism of the present invention, the high-refractive-index layer and low-refractive-index layer preferably have optical film thicknesses of 0.29xcex/4-1.25xcex/4 and 0.29xcex/4-2.25xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
In the prism of the present invention, the first antireflection film may include an odd number of layers with the low-refractive-index layer arranged immediately on the first face of the base member and the low-refractive-index layer arranged as the outermost surface of the first antireflection film. Alternatively, the first antireflection may include an even number of layers with the high-refractive-index layer arranged immediately on the first face of the base member and the low-refractive-index layer as the outermost surface of the first antireflection film.
When the first antireflection film includes an even number of layers, preferably, it has any of the following structures to maximize transmittance.
(1) The first antireflection film includes first and second layers successively formed on the base member side. The first and second layers have optical film thicknesses of 0.88xcex/4-0.95xcex/4 and 0.29xcex/4-0.32xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(2) The first antireflection film includes first to fourth layers successively formed on the base member side. The first to fourth layers have optical film thicknesses of 1.15xcex/4-1.22xcex/4, 1.37xcex/4-1.40xcex/4, 0.88xcex/4-0.96xcex/4, and 0.29xcex/4-0.32xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(3) The first antireflection includes first to sixth layers successively formed on the base member side. The first to sixth layers have optical film thicknesses of 1.02xcex/4-1.22xcex/4, 1.39xcex/4-1.47xcex/4, 1.08xcex/4-1.22xcex/4, 1.39xcex/4-1.48xcex/4, 0.84xcex/4-0.95xcex/4, and 0.29xcex/4-0.32xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(4) The first antireflection film includes first to eighth layers successively formed on the base member side. The first to eighth layers have optical film thicknesses of 0.34xcex/4-1.21xcex/4, 1.40xcex/4-1.70xcex/4, 0.80xcex/4-1.22xcex/4, 1.40xcex/4-1.80xcex/4, 0.79xcex/4-1.22xcex/4, 1.40xcex/4-1.81xcex/4, 0.70xcex/4-0.95xcex/4, and 0.29xcex/4-0.32xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(5) The first antireflection film includes first to tenth layers successively formed on the base member side. The first to tenth layers have optical film thicknesses of 0.34xcex/4-1.20xcex/4, 1.41xcex/4-1.94xcex/4, 0.45xcex/4-1.21xcex/4, 1.41xcex/4-2.20xcex/4, 0.49xcex/4-1.21xcex/4, 1.42xcex/4-2.09xcex/4, 0.58xcex/4-1.20xcex/4, 1.42xcex/4-1.99xcex/4, 0.63xcex/4-0.94xcex/4, and 0.29xcex/4-0.32xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
In the prism of the present invention, the first antireflection film may include an odd number of layers with the low-refractive-index layer arranged immediately on the first face of the base member and the low-refractive-index layer arranged as the outermost surface of the first antireflection film. Alternatively, the first antireflection may include an even number of layers with the high-refractive-index layer arranged immediately on the first face of the base member and the low-refractive-index layer arranged as the outermost surface of the first antireflection film.
The second antireflection film preferably includes any of the following structures to maximize transmittance.
(a) The second antireflection film includes first and second layers successively formed on the base member side. The first and second layers have optical film thicknesses of 0.95xcex/4-1.01xcex/4 and 0.98xcex/4-1.04xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(b) The second antireflection film includes first to third layers successively formed on the base member side. The first to third layers have optical film thicknesses of 1.38xcex/4-1.46xcex/4, 0.80xcex/4-0.84xcex/4, and 0.96xcex/4-1.02xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(c) The second antireflection includes first to fourth layers successively formed on the base member side. The first to fourth layers have optical film thicknesses of 0.66xcex/4-0.70xcex/4, 0.79xcex/4-0.83xcex/4, 0.66xcex/4-0.70xcex/4, and 1.30xcex/4-1.35xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
(d) The second antireflection film includes first to fifth layers successively formed on the base member side. The first to fifth layers have optical film thicknesses of 2.07xcex/4-2.19xcex/4, 0.33xcex/4-0.35xcex/4, 1.55xcex/4-1.65xcex/4, 0.76xcex/4-0.80xcex/4, and 0.98xcex/4-1.04xcex/4 with respect to any design basis wavelength xcex of at most 200 nm, respectively.
Note that the above defined range for optical film thickness of each layer is determined taking into account a thickness controlling range for practical film formation with respect to optimum design thickness that may provide optimum transmittance.
According to a preferred embodiment of the prism of the present invention, the first antireflection film includes any number of layers selected from a group of two, four, six, eight, and ten layers with the high-refractive-index layer arranged immediately on the first face of the base member and the low-refractive-index layer arranged as the outermost surface of the first antireflection film. The second antireflection film includes any number of layers selected from a group of two, three, four, and five layers with the low-refractive-index layer arranged as the outermost surface of the second antireflection film.
To maximize the effect of the present invention, the prism of the present invention is preferably a right-angle prism.
Further, to maximize the effect of the present invention, the prism of the present invention is preferably used such that light with incident angle of 65-80xc2x0 is incident upon the first face of the base member and light with incident angle of 0xc2x0 is incident upon the second face of the base member.
An optical device according to another aspect of the present invention uses the above described prism as an optical element for narrowing an excimer laser beam with a wavelength of at most 200 nm. The optical device is preferably a laser resonator. Generally, at least three prisms are used for narrowing wavelength band in a grating method. Thus, if transmittance per prism is increased by 1-2%, efficiency (transmittance) of the narrowing unit increases by 3-6%, which would be a considerable contribution to a laser output. As a result, a laser input value can be decreased, whereby the useful life of components in the resonator, such as an optical component, electrode and laser gas, may be prolonged, and a laser beam may be effectively narrowed.
According to the present invention, the prism with higher transmittance than the conventional prism can be provided. By forming the outermost surface of the antireflection film with aluminum fluoride, the prism may be provided with excellent environmental resistance. Further, the optical device using the prism of the present invention restrains an output loss of light. For example, the use of the prism of the present invention in the resonator for narrowing the wavelength band of excimer laser beam enables effective narrowing of the band without lowering laser energy efficiency.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.